Orthopaedic implants are known to have at least a portion of an outer surface thereof be porous. A porous surface of an orthopaedic implant may be used in order to anchor the implant in place once it is implanted in the body. Preferably, a porous surface promotes ingrowth of surrounding bone tissue after implantation of an orthopaedic implant at a surgical site. Bone cement may also be introduced to the surgical site and enter the porous surface of an orthopedic implant to further anchor the implant in a preferred location and/or orientation.
Various manufacturing methods have been developed to create a porous surface for orthopaedic implants. The porosity of a porous surface may be manufactured based on the characteristics of the surrounding bone tissue at the surgical site or the viscosity of bone cement that may be applied to the implant. For instance, an implant having an outer surface with larger void spaces may be preferable for a bone cement having a higher viscosity because such a bone cement will have an easier time entering into the larger void spaces before solidifying.
In some applications, it is beneficial to have a porous surface with a particular configuration. Selective melting (SLM) using either a laser or electron beam can be used to create a porous surface having a pre-planned structure. During the SLM process, heat fusable powder is generally deposited one layer at a time into a container adapted to house the powder. For each layer, a moving energy beam is used to melt the powders in certain areas corresponding to the pre-planned geometry of a component being manufactured. In SLM, the energy beam is directed by a computer aided design (CAD) solid model of the component being fabricated. Layer by layer, the powders are gradually joined into a solid mass that forms a three-dimensional geometry. In areas not struck by the laser beam, the powders remain loose. The loose powders serve to support the solid regions of the component as the fabrication proceeds. At the completion of the process, the fabricated component may be removed from the container and the loose powders generally remain in the container.
Other methods of fabricating both porous and solid structures include traditional powder metallurgy (PM) processes. A typical PM process can include consolidation of powder with or without binding agents and/or soluble pore forming agents. The consolidated part is known as a “green” part, and can be shaped prior to subsequent processing steps. The subsequent processing steps can include removal of the pore forming agents and sintering.
For some manufacturing methods it is necessary to attach a fabricated porous structure to an implant that acts as a substrate for the porous structure. In order for a porous surface of an orthopaedic implant to function as desired once implanted, the structure of the porous surface should preferably be uncompromised. For example, maintaining the integrity of a porous surface is generally an important consideration in both the manufacture thereof and the bonding thereof to a substrate layer in the form of an orthopaedic implant. It is a further important consideration to bond a porous layer to a substrate layer without significantly negatively affecting the underlying mechanical properties of the substrate layer.
Sintering may cause material fragments, whether thermoplastic or metal, to fuse. In addition, any mechanical pressure used to hold the porous structure and substrate in intimate contact during sintering can also serve to deform and distort the porous structure. The resultant altered porosity of the orthopedic implant may inhibit the desired tissue ingrowth capability of the porous layer. Additionally, sinter bonding may rapidly degrade the mechanical properties of an underlying substrate due to either grain growth and/or phase transformation resulting in a change in grain and phase morphology. This issue is a particular problem for some forged or cast substrates. In particular for the titanium alloy Ti6Al4V, sinter bonding occurs above the Beta-transus, which may cause both a rapid increase in the beta grain size and a notable change in the shape of the alpha/beta distribution after cooling to room temperature. As a result of both, the fatigue properties of the Ti6Al4V may be significantly negatively affected. This limits the applications for which sinter bonding is appropriate to those with low fatigue requirements.
Traditional diffusion bonding of material layers generally requires heating of the layers to be bonded together and applying a force to the interface. Generally, the force is applied in a uniaxial or biaxial fashion and requires complex and specific fixturing for different component geometries (i.e. orthopaedic implants which may not be flat and/or uniplanar) and/or sizes with which to apply the uniaxial or biaxial forces. Such fixturing is generally specific to component size and/or geometry. In addition, traditional diffusion bonding generally applies force over a broad area on the outer surface of a porous layer. The pressure exerted on the outer surface of the porous layer generally needs to be limited in order to limit material deformation. The higher the pressures used during traditional diffusion bonding, the more deformation is caused.
There is therefore a need for a reliable and economical method to attach porous structures to substrates with a complex geometry.